Neonchameleon
Legend
You may barely be talking about the DM, but there are DMs talking about players; some of the DMs are claiming not to be control freaks, while I'm admitting I kinda am one. My own suspension of disbelief matters to me while I'm DMing--I am tempted to say I can't DM without it. That's part of the reason I'm homebrewing the world--I have a harder time managing suspension of disbelief in the worlds others create.
So, the folk I don't allow, I don't allow because they don't make sense to me, in the world I'm making. I am not intending to accuse any player who wants to play something I'm not allowing of anything worse than possible tone-deafness. There is no "purity of the world" to be violated, there is no judgment of playstyle.
My attitude when DMing is pretty much the literal opposite of this. The real world is not just stranger than I imagine, but stranger than I can imagine. I've limited knowledge and was born in a time, place, and part of the world. Both humans and evolution frequently take ideas and push them past any sort of sane or sensible limits as they get competitive. Add magic to the mix and things get even weirder, especially if "a wizard did it" is a possible answer as to why. As DM I know the major players - but there are too many minor players to grasp.
I might not be willing to add The Great Tabaxi Empire to a D&D inspired setting (or I might put it on the far side of the world) but the idea that a wizard created a family of catpeople a couple of generations back because they could and because they wanted to? Of course and I'd be surprised if several wizards didn't. Because wizards. And because people. It's more setting-breaking for me if no wizard ever tried to do this unless there is some specific magical law that prevents it working.
I have two hard limits - without direct divine intervention societies aren't static, and without direct divine intervention pure dystopias fall apart. I've yet to decide whether Menzobaranzan is essentialy Lolth playing The Sims with a lot of direct intervention or whether it's a potemkin city for Lolth's benefit. Either way it's not natural. (I also in the setting I'm writing have no winged humanoids who can use their wings to fly due to the square cube law). But a simple rule for me of setting building is if it is possible within the bounds of the setting some idiot will have tried it - and as DM I can't anticipate everything even my players will try, never mind all the NPCs.